1. Field of the Invention
The present invention generally relates to aircraft power systems, and more particularly to an improved system for regulating the voltage on a direct current (d.c.) aircraft power bus.
2. Description of the Prior Art
Direct current electrical power supply systems proposed for use in aircraft comprise an alternating current (a.c.) generator, a full-wave rectifier, and a filter capacitor. A bus connects the d.c. voltage across the filter capacitor to the load which varies over a wide range. In such prior art systems, the d.c. voltage across the capacitor is sensed. A voltage regulator compares the sensed voltage with a reference and controls a d.c. exciter drive to maintain a constant d.c. output voltage.
As mentioned above, the resistive load in an aircraft varies over a wide range. In addition, as will be appreciated by those skilled in the art, the frequency of the a.c. generator also varies owing to the fact it is driven by the aircraft propulsion unit. The response of the overall system to the signal fed back depends on magnitude of the load owing to the resistive-capacitive network formed by the load and the filter capacitor. For large loads (low resistance--high current) the overall system response is relatively fast. In this high current situation the gain of the voltage regulator can be high. However, when the resistance is high and the current is low, system response is relatively slow, and if the regulator gain is high the system may be unstable. In addition, the system gain is also a function the rotational speed of the a.c. generator; as its speed increases the system gain increases. The voltage regulator gain should change as the rotational speed of the a.c. generator changes in order to maintain the overall open loop system gain constant.
U.S. Pat. No. 4,807,106 is an example of a d.c. aircraft power supply. That invention provides a method for regulating a d.c. electrical power source having a generator, with an exciter field coil, that produces an a.c. output voltage which is rectified to produce a d.c. output voltage. The method includes the steps of: producing a first signal proportional to the a.c. output voltage of the generator; sensing the current in the exciter field winding of the generator; producing a second signal proportional to the exciter field current; and combining the first and second signals to produce a third signal. A fourth signal proportional to the d.c. output voltage of the power source is compared to a d.c. reference signal to produce a trim error signal. The trim error signal is combined with an a.c. reference signal to produce a fifth signal. The third and fifth signals are compared to produce a sixth signal and the current in the exciter field winding is controlled in response to this sixth signal.